FIG. 4 shows a configuration of a conventional servo accelerometer of this type (for example, reference 1: Japanese Patent Application Laid-Open No. H11-281670), and FIG. 5 show a configuration of a pendulum part 11 of the servo accelerometer.
As shown in FIG. 5, a circular pendulum part 11 is composed of a frame body 11A having a substantially annular shape, notches 11B and 11C, pendulum 12 having a substantially circular and/or tongue-like shape and a diameter smaller than the inner diameter of the frame body 11A, and a pair of hinges 13 that connect the pendulum 12 and the frame body 11A to each other and support the pendulum 12 in the frame body 11A in such a manner that the pendulum 12 can swing in the thickness direction thereof. The frame body 11A, the pendulum 12 and the hinges 13 are integrally formed of quartz, for example, to constitute the pendulum part 11. The hinges 13 are thin and can be elastically deformed.
As shown in FIG. 4, the pendulum part 11 is interposed between a pair of magnetic yokes 14, 15, and the opposite surfaces of the frame body 11A of the pendulum part 1 abut on the magnetic yokes 14, 15, respectively. Both the magnetic yokes 14, 15 have the shape of a circular cup having one end open and the other end closed, and open ends 14c and 15c thereof abut on the frame body 11A. The magnetic yokes 14, 15 and the pendulum part 11 are fixed to each other by adhesive. The magnetic yokes 14, 15 are made of a metal material, such as Invar, which has a low coefficient of thermal expansion, for example.
The magnetic yokes 14, 15 house pole piece bottoms 16′, 16, permanent magnets 17′, 17 and pole piece tops 18′, 18, respectively. The pole piece bottoms 16′, 16, the permanent magnets 17′, 17 and the pole piece tops 18′, 18 have a disk-like shape and are successively stacked on closure plate portions 14a, 15a of the magnetic yokes 14, 15, which constitute the closed ends of the magnetic yokes 14, 15, respectively, with the center axis thereof aligned with the center axis of the magnetic yokes 14, 15. The pole piece tops 18′, 18 have a thickened peripheral edge, as shown in FIG. 4.
The permanent magnets 17′, 17 may be rare-earth cobalt magnets, such as samarium cobalt magnet. The pole piece bottoms 16′, 16 and the pole piece tops 18′, 18 may be made of electromagnetic soft iron. The permanent magnets 17′, 17 are fixed to the pole piece bottoms 16′, 16 and the pole piece tops 18′, 18 by adhesive. The pole piece bottoms 16′, 16 are fixed to the magnetic yokes 14, 15, respectively, by laser welding. In FIG. 4, reference numerals 14b, 15b denote through-holes formed in the closure plate portions 14a, 15a, respectively, for the purpose of laser welding.
The permanent magnets 17′, 17 are magnetized in the thickness direction thereof and form annular magnetic gaps 19′, 19 between the inner surface of the open end of the magnetic yokes 14, 15 and the outer surface of the pole piece tops 18′, 18, respectively.
The magnetic yokes 14, 15 and the pole piece bottoms 16′, 16, the permanent magnets 17′, 17 and the pole piece tops 18′, 18 housed in the magnetic yokes 14, 15 constitute yoke parts 26′, 26, respectively.
Cylindrical torque coils 21′, 21 are attached to the opposite surfaces of the pendulum 12. The torque coils 21′, 21 are wound around bobbins 22′, 22. The bobbins 22′, 22 have end plates 22a′, 22a at the end closer to the pendulum 12, and cylindrical attachment parts 22b′, 22b are formed at the center of the end plates 22a′, 22a. 
The bobbins 22′, 22 with the torque coils 21′, 21 wound therearound are attached to the pendulum 12 by fixing cylindrical holders 23′, 23 to the opposite surfaces of the pendulum 12 by adhesive, fitting the attachment parts 22b′, 22b of the bobbins 22′, 22 into the holders 23′, 23, and fixing the attachment parts 22b′, 22 to the holders 23′, 23, respectively, by adhesive. As shown in FIG. 4, small gaps G1′, G1 are formed between the end plates 22a, 22a of the bobbins 22′, 22 and the opposite surfaces of the pendulum 12. As with the pendulum 12, the holders 23′, 23 are made of quartz.
As shown in FIGS. 5, on the opposite surfaces of the pendulum 12, arc-shaped electrodes 24′, 24 are disposed at a position on the outer side of the torque coils 21′, 21. The magnetic yokes 14, 15 have electrode surfaces 14e, 15e that face the electrodes 24′, 24, respectively. Thus, As shown in FIG. 6, the surfaces of the open ends of the magnetic yokes 14, 15 include frame body abutting surfaces 14c, 15c, recesses 14d, 15d, and the electrode surfaces 14e, 15e facing the areas of the pendulum 12 where the electrodes 24′, 24 are formed, respectively, arranged in this order from the outer periphery thereof.
The frame body abutting surfaces 14c, 15c of the magnetic yokes 14, 15 are adhered to the opposite surfaces of the frame body 11 A of the pendulum part 11 to sandwich the pendulum part 11 , so that a single piece of body of the sensing mechanism 10 is obtained. In other words, the yoke parts 26′, 26 and the pendulum part 11 are integrated. Once these components are integrated, the cylindrical torque coils 21′, 21 are disposed in the annular magnetic gaps 19′, 19 in the magnetic yokes 14, 15, respectively. Furthermore, the electrode surfaces 14e, 15e face the electrodes 24′, 24 with predetermined gaps G2′, G2 interposed therebetween, respectively.
An outer ring 25 is mounted astride the outer surfaces of the magnetic yokes 14, 15 and makes the magnetic yokes 14, 15 electrically continuous. As with the magnetic yokes 14, 15, the outer ring 25 is made of Invar and fixed to the magnetic yokes 14, 15 by a conductive adhesive. In this way, the integral sensing mechanism 10 is provided and housed in a housing 30.
The housing 30 protects the sensing mechanism 10, serves an acceleration detection function and serves as an attachment mechanism for attachment to a target object for acceleration detection (a moving object). In this example, the housing 30 has the shape of a cylindrical cup with one end closed. The housing 30 has a flange 31 for attachment at the open end thereof, and a side surface (lower surface) of the flange 31 serves as an attachment surface 31a. The housing 30 is made of a stainless material, for example.
A C-ring 41 is fitted around the magnetic yoke 15, and the sensing mechanism 10 is fixed to and supported on the housing 30 via the C-ring 41. The C-ring 41 is fixed to the magnetic yoke 15 and the housing 30 by adhesive.
Similarly, a C-ring 42 is fitted around the magnetic yoke 14 and fixed to the magnetic yoke 14 by adhesive. A space between the C-ring 42 and the housing 30 is filled with a flexible adhesive 43 of silicone resin or the like. The C-rings 41, 42 are disposed to stable the sensing mechanism 10. The C-rings 41, 42 are both made of aluminum. To protect the inner components, the open end of the housing 30 may be appropriately covered with a lid plate.
The servo accelerometer thus configured detects a displacement of the pendulum 12 in the thickness direction caused by input of acceleration as a variation in capacitance, which results from a variation of the gap G2′ between the electrode 24 and the electrode surface 14e or the gap G2 between the electrode 24′ and the electrode surface 15e (no electric signal paths are shown). The electrode surfaces 14e, 15e are connected to common GND, detection signals from the electrodes 24′, 24 on the opposite surfaces of the pendulum 12 are differentially amplified by a required electrical circuit (not shown), and an electric current produced according to the difference in capacitance is supplied to the pair of torque coils 21′, 21. Interaction between the electric current flowing through the torque coils 21′, 21 and the magnetic field of the permanent magnets 17′, 17 causes the pendulum 12 to return to the original position and come into equilibrium at the neutral point. Because the electric current is proportional to the acceleration applied to the pendulum 12, the acceleration can be determined from the electric current.
In the conventional servo accelerometer configured as described above, the sensing mechanism 10 is fixed to and supported on the housing 30 via the C-rings 41, 42 fitted around and fixed by adhesive to the magnetic yokes 14, 15.
Thus, for example, if a stress is exerted on the attachment surface 31a of the housing 30 from a target structure (a target object for acceleration detection) because of a variation in ambient temperature, for example, the stress causes deformation of the magnetic yokes 14, 15. In addition, if a thermal stress occurs in the housing 30, the stress is transferred to the magnetic yokes 14, 15 through the C-rings 41, 42 to cause deformation of the magnetic yokes 14, 15.
Such deformation of the magnetic yokes 14, 15 results in displacement of the pendulum 12 from the neutral position, thereby causing a variation in bias. Thus, the bias stability is degraded.
In the process of transfer of the stress from the housing 30 to the magnetic yokes 14, 15 via the C-rings 41, 42, an excessive stress is exerted on the adhesive layer between the housing 30 and the C-rings 41, 42 and the adhesive layer between the C-rings 41, 42 and the magnetic yokes 14, 15. In addition, the deformation of the magnetic yokes 14, 15 causes an excessive stress exerted on the adhesive layer between the magnetic yokes 14, 15 and the frame 11 supporting the pendulum 12. As a result, cracking or peel-off of these adhesive layers occurs.
Such cracking or peel-off of these adhesive layers leads to a variation in stress distribution in the sensing mechanism 10, which is a major cause of an irreversible bias variation in an environment where the temperature varies.
In view of such circumstances, an object of the present invention is to provide a servo accelerometer that achieves a high bias stability, is reduced in number of causes of an irreversible bias variation, and has excellent temperature characteristics.